This section describes background subject matter related to the disclosed embodiments of the present invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art.
Advanced lithography requires nanometer-scale dimensions for the next generation ICs, data storage, LEDs and Solar cells. The sub-100 nm features can be patterned successfully using only very thin resists—nanoresists (<10 nm). Moreover, nanoresists are needed to realize full capability of high resolution lithography, like one using low energy beams patterning or near-field optical lithography. The most widely used method of photoresist deposition is spin-coatings. It involves dispensing controllable amount of photoresist in liquid form onto a rotatable substrate. Photoresist is spreading across the surface of the substrate due to centrifugal forces. For nanoresist thicknesses (<10 nm), spin coating becomes a significant challenge, not only from achieving uniform and reproducible nanothickness using correct and extensive solvent dilution, but also inability to overcome regular defects associated with spin-coating technique: striations, edge bead and streaks. Also, spinning of non-round substrate materials create additional problems with uniformity.
There are known methods of photoresist deposition by roller applicators, for example Roller-coating system RC-4000 from HTP HiTech Photopolymere AG or System suggested by Hein in U.S. Pat. No. 6,344,087. These methods are designed to deposit only relatively thick (microns range) photoresists for Printed Circuit Boards (PCB) patterning.
Nanometrix company has developed a system for nanoresist deposition base on so called continuous coating process, named Schneider-Picard (SP). Method is performed by sliding elements on the surface of a liquid until they meet the ultra thin film formation line. At the same rate the elements are being deposited onto the interface and packed at the formation line, the monolayer is transferred from the liquid surface onto a solid substrate. The whole process works in a dynamic and continuous equilibrium. During the SP method polymer solution is applied at the gas-liquid interface. The receiving phase or the sub-phase is a moving flat liquid. After injection, the polymer solution thins down in different ways depending on the physico-chemical characteristics of the solvents and liquids. Evaporation and immersion are the main ways in which the solvent concentration, after spreading the polymer, fades down from the gas-liquid inter-phase. The pressure applied onto the film's long axis is kept constant while a conveyer transfers the film from the liquid surface toward the solid substrate.
Organic thin films, usually polymers, have been traditionally applied as photoresists for lithography since they can be modified or structured with energetic beams that use photons, electrons, or ions. However, the obtained resolution for such resists is limited by their relatively large thicknesses and intermolecular distances.
For the next generation lithography techniques with expected lateral resolution in the nanometer scale, Self-Assembled Monolayers (SAM) have attracted great attention. SAMs are ordered molecular assemblies formed spontaneously by chemisorption of molecules with suitable anchor groups on a solid surface. They have a stable, homogeneous, oriented, and well ordered molecule structures with a typical thickness of a few nanonieters and an intermolecular spacing on the order of 0.5 nm. Because of their nanometer size thickness and sub-nanometer intermolecular distance, SAMs potential resolution is higher than for polymer resists. Sub-100 nm lithography of SAM can be achieved by soft imprinting lithography, scanning-probe microscopy (SPM) that includes dip-pen or anodic oxidation nanolithography, or by energetic beams, like X-ray or electron beam lithography, or photolithography.
The following 2 methods of SAM deposition are widely used: Liquid phase deposition (using SAM solutions) and Vapor phase deposition (CVD). The first method involves dipping a substrate in a bath contained SAM solution, the second method involves placing a substrate in a vacuum chamber and exposing it to a vapour SAM molecules evaporated from a precursor. Both methods are not suited for coating a large substrates or continuous films during roll-to-roll operation.
Whitesides et al suggested the method of patterning of self-assembled momonlayers using so called, microcontact printing (uContact printing). This method is based on fabricating a mold having desired pattern as a surface relief structure, wetting this relief features with SAM and contacting such mold with the substrate. During such contact SAM molecules are being transferred from the mold onto the substrate material only at the places of the contact, which creates a pattern. This method has limit in resolution caused by molecules transfer in a vapor phase onto the areas between feature, which rapidly washes out the pattern.
SAMs have been successfully patterned using standard photolithography with a photomask. Hiroyuki Sugimura et al. from Nagoya University has reported patterning with features as low as 2 um using UV excimer light at 172 nm and a photomask placed in a contact with SAM.
The same Hiroyoki Sugimura has published Vacuum UV exposure system operating at <200 nm wavelength, which is used in patterning SAM in proximity gap <1 um between a photomask and the SAM.
Though regular optical lithography is limited in resolution by diffraction effects some new optical lithography techniques based on near field evanescent effects have already demonstrated advantages in printing sub-100 nm structures, though on small areas only. Near-field phase shift lithography NFPSL involves exposure of a photoresist layer to ultraviolet (UV) light that passes through an elastomeric phase mask while the mask is in conformal contact with a photoresist. Bringing an elastomeric phase mask into contact with a thin layer of photoresist causes the photoresist to “wet” the surface of the contact surface of the mask. Passing UV light through the mask while it is in contact with the photoresist exposes the photoresist to the distribution of light intensity that develops at the surface of the mask. In the case of a mask with a depth of relief that is designed to modulate the phase of the transmitted light by π, a local null in the intensity appears at the step edge of relief. When a positive photoresist is used, exposure through such a mask, followed by development, yields a line of photoresist with a width equal to the characteristic width of the null in intensity. For 365 nm (Near UV) light in combination with a conventional photoresist, the width o the null in intensity is approximately 100 nm. A PDMS mask can be used to form a conformal, atomic scale contact with a flat, solid layer of photoresist. This contact is established spontaneously upon contact, without applied pressure. Generalized adhesion forces guide this process and provide a simple and convenient method of aligning the mask in angle and position in the direction normal to the photoresist surface, to establish perfect contact. There is no physical gap with respect to the photoresist. PDMS is transparent to UV light with wavelengths greater than 300 nm. Passing light from a mercury lamp (where the main spectral lines are at 355-365 nm) through the PDMS while it is in conformal contact with a layer of photoresist exposes the photoresist to the intensity distribution that forms at the mask.
Our earlier patent application PCT/US2008/012901 has described high-throughput large area Near-field optical lithography method, which is based on “rolling mask” technique. One of the embodiments involves cylindrical-shaped transparent mask with elastomeric nanostructured film (phase mask) laminated on the surface of such cylinder, and a source of UV light inside such cylinder. Such “rolling mask” is brought in contact with the photosensitive layer and then rolled over the surface of the layer. During this dynamic near-field exposure sub-100 nm features have been written in photoresist. To take full advantage of the near-field optical patterning technique it is desirable to use very thin photoresists. SAMs as monomolecular and conformal layers having 100% uniformity are very promising choice.
Graham Leggett described in US patent application 20050048411 using SAMs as resists for Scanning Near-filed optical lithography (SNFOL). In the case of scanning near-field lithography a narrow optical fiber (having an internal diameter as small as 50 nm) is brought in close proximity to a sample surface. Under such conditions, as result of near-field effect, light may be transmitted through the aperture without diffraction. He demonstrated patterning SAM layers with resolution down to 25 nm using this technique. The downside of the technique is that it is a sequential method (direct writing), so throughput of patterning is very low.